Is the technique sensitive to sub-monolayer changes in surface composition, or is it more appropriate for studying "thicker" thin films?
Indeed it is sensitive to the top three atomic layers and depending on the work function change between substrate and film, can see sub mono-layer coverage.
Professor Baikie's experience lies mostly with oxidation and in his experience with Oxygen adsorption on clean Silicon surface claimed 1/1000 of a monolayer. It is the most sensitive technique available, certainly more sensitive in some cases than Auger Electron Spectroscopy (AES) and much more so than XPS (X-ray Photoelectron Spectroscopy).
When additional parameters are required the KP technique can be used as a supplement to these other techniques. The spectroscopies give a "finger print" of the sample not just one parameter. However take for example a UHV application - The UHV-KP is very good for scoping out changes in the surface during an experiment and then suggesting where to look for critical changes in behaviour. If you used UHV scanning probes such as AFM or SMT this could take weeks or months. With KP possibly 1 hour.